1. Field of the Invention
The invention relates to an integrated circuit with a test circuit for checking circuit-internal signals with the aid of an external tester device. The invention furthermore relates to a method for checking a function in an integrated circuit. The invention furthermore relates to a test system with an integrated circuit and a tester device for checking a function in the integrated circuit.
2. Description of the Related Art
During the testing of integrated circuits, it is necessary to detect internal signals and forward them to a tester device for checking. The internal signals can be tapped off on the one hand via connection pads of the integrated circuit or by placing measuring tips on connecting lines of the integrated circuit. The number of available connection pads is limited on an integrated circuit since said connection pads take up a large area in comparison with the circuit structures. On the other hand, signals detected by measuring tips are measurable only with a large error tolerance being accepted, since the driver power of the signals between the gate circuits of the integrated circuit usually does not suffice to sufficiently compensate for the inductive load of a measuring tip or of the contact between measuring tip and interconnect. Equally, in the case of analog signals, the resistance of the measuring tip, of the tester line with respect to the measuring tip and/or of the contact between measuring tip and interconnect is beset with an unknown resistance which corrupts the measurement of currents or voltages.
In the case of very weak signals, in particular, the measurement with an external tester device leads to unusable results, irrespective of whether the measured signal is measured via a connection pad or by emplacement of a measuring tip.
Hitherto, test mode circuits have been provided in the integrated circuit, which check the internal signals or internal signal relationships and compare them with corresponding desired values in order to check the circuit in accordance with its specification. One disadvantage of the test mode circuits is the large space requirement in the integrated circuit, since each test mode circuit has to be connected to a control circuit with an interconnect.
A further disadvantage of measuring internal signals by means of an externally connected tester device is that the signal changes in some instances take place so rapidly that the external tester device cannot ascertain them on account of capacitances and inductances.